1. Field of the Invention
The invention relates in general to elevator systems, and more specifically to the enhancement of traction in a traction type elevator system to permit the use of a lighter elevator car and counterweight.
2. Description of the Prior Art
Reduction in the dead weight of the elevator car is desirable from a cost viewpoint. In addition to cost savings in the elevator car itself, it enables savings to be made in associated items such as the counterweight, guide rails, safety, traction drive machine, and the building support structure. A lighter elevator car reduces power requirements from the building, as lower peak torques and thus lower currents are required.
A limiting factor in reducing the weight of the elevator car and counterweight however, is the tractive force between the ropes and drive sheave. Sufficient tractive force or traction must be available over the complete range between no-load or empty car, to full load, for the desired acceleration and deceleration rates, or slippage will occur.
The tractive force between the ropes and sheave is governed by the relationship: EQU T1/T2=e.sup..mu..theta.
where:
T1=rope tension on the car side of the drive sheave PA1 T2=rope tension on the counterweight side of the drive sheave PA1 e=base of natural logarithms PA1 .mu.=the effective coefficient of friction between the rope and drive sheave for the groove geometry employed PA1 .theta.=angle of wrap or contact between the ropes and drive sheave.
In the prior art, sheave grooves are often undercut to increase the effective coefficient of friction, but only so much can be done in this regard as the resulting increased pressures on the ropes and sheave grooves shorten both rope and sheave life. Special high traction lubricants have been applied to the ropes to increase the effective coefficient of friction. Sheave grooves have also been lined with a treaded elastomeric material, and arrangements have been made which press the ropes more tightly into the sheave grooves. Other arrangements for increasing tractive force relate to increasing the angle of wrap by going from the half or single wrap to a full or double wrap, and even to a 270 degree wrap. Increasing the wrap, however, increases the bearing load on the drive sheave bearings, requiring a larger and more costly traction drive machine. The 270 degree wrap erodes the sides of the grooves because of the turning and tilting of the drive components required in order to prevent interference between the ropes.